Coaxial dual-unit electrodynamic loud-speaker



Jan. 30, 1951 H. F. OLSON EI'AL comm. DUAL-UNIT ELECTRODYNAMIC LOUD-SPEAKER Filed April 29. 1949 INVENTORS HARRY F. ElLsmN 3g 1' m-m PR5 TEN ATTORNEY Patented Jan. 30, 1951 COAXIAL DUAL-UNIT ELECTRODYNAMIC LOUD-SPEAKER Harry F. Olson, Princeton, and John Preston,

Metedeconk, N. J assignors to Radio Corporation of America, a corporation of Delaware Application April 29, 1949, Serial No. 90,464

4 Claims.

Our present invention relates to signal translating apparatus, and more particularly to an improvement in dynamic loudspeakers of the duo-cone type.

In the field of sound reproduction, the duocone, dynamic loudspeaker has played an important part in extending the frequency response range over which sound may be reproduced efficiently. Among the diiferent types of duo-cone loudspeakers which have been designed and built in the past is that which is particularly described in U. S. Patent 2,426,948, issued to applicant J. Preston on September 2, 1947. In this loudspeaker, the high frequency cone is coaxially mounted within the low frequency cone, and both cones are driven by separate voice coils disposed in a magnetic field, the system operating in a well known manner. A loudspeaker such as that disclosed in the above-mentioned patent, while having a much higher high frequency response than conventional, single-cone loudspeakers, is quite expensive, however. In an effort to provide an improved loudspeaker of this type, we have made certain changes and improvements which result in a simpler and less expensive construction. Thus, one of the objects of our present invention is to provide an improved, simplified, more efiicient construction for a loudspeaker of the duo-cone type.

t is another object of our invention to provide an improved construction for the vibratory system of a duo-cone loudspeaker.

It is also an object of our present invention to provide an improved dynamic type loudspeaker which will cover an extended range of useful. audio frequencies over that covered by conventional single-cone loudspeakers, yet which will be less expensive than a duo-cone loudspeaker such as that disclosed in the above-identified Patent No. 2,426,948.

It is still another object of our present invention to provide a duo-cone loudspeaker of the dynamic type which is simple in construction, highly efficient in use, and which can be produced at a minimum of cost.

In accordance with our present invention, we provide a loudspeaker with two separately driven diaphragms, one a large, cone-type diaphragm driven by a large voice coil for reproducing signals in the low frequency range, and the'other a small, cone-type diaphragm driven by a small voice coil for reproducing signals in the high frequency range. The cones and voice coils are disposed in concentric, nested, overlapping,

spaced relation-with the-base of the small cone coupled to the inside surface of the large cone by a suitable compliant coupling. The small ends of both cones are supported from the magnetic structure by suitable, compliant, suspension members which also serve to center the voice coils in the air gaps provided in the magnetic structure. The base of the large cone is suspended, in well known manner, from a dish-pan type support attached to the magnetic structure. For the purpose of preventing vibrations of the large cone from being transmitted to the small cone, the compliance of the center suspension of the small cone is small and relatively stiff compared to the ccmpliance of the base suspension of the small cone. In addition, vents are provided in the large cone for the space between the small and large cones. A bridge-type magnetic structure, similar to that disclosed in our copending application, Serial Number 51,962, filed September 30, 1948, makes it possible to supply the two air gaps from a single magnet.

The novel features of our invention, as well as additional objects and advantages thereof, will be better understood from the following detailed description of a single embodiment thereof when read in connection with the accompanying drawing, in which Figure 1 is a side view in section, taken through the center, of a duo-cone loudspeaker in accordance with our present invention,

, Figure 2 is an enlarged plan view in section, taken on the, line 2--2 of Figure 1,

Figure 3 is a wiring diagram of an electrical circuit corresponding to the mechanical network of the vibrating system of the loudspeaker shown in Figure 1, and

Figure 4 is a curve showing the frequency response characteristic of a duo-cone loudspeaker in accordance with our present invention and mounted On a fiat baifie.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown a loudspeaker system having a pair of nested, overlapping, truncated cones l, 3 which are mounted in concentric, spaced relation. The outer cone I is comparatively large and is particularly adapted for reproducing low frequency signals, while the inner cone 3 is'comp'ar'atively small and is particularly adapted for reproducing high frequency signals. The cones l, 3 are connected, respectively, at their apex ends to voice coil forms 5, E which have voice coils 1,.8 dis posed, respectively, on their outer surfaces. j

For producing a magnetic field, there is provided a suitable magnetic structure 9 which may be of the type disclosed and claimed in'diii aboveidentified copending application which comprises a yoke H and a front plate it attached to the ends of the yoke, the front plate having a central, annular aperture i5 therein. A permanent magnet IT has one end is thereof mounted on the base 19 of the yoke ll with a solid, central cylindrical pole piece 2i disposed on the opposite end 23 thereof and extending through the aperture I 5. Also mounted on the end 23 is another cylindrical pole piece 25 which is disposed in spaced relation to the pole piece 211 so as to provide an inner annular air gap 2'! therebetvveen. The pole piece 25 extends through the front plate aperture l5 also in spaced relation to the front plate 13 so as to provide an outer air gap 25 therebetween. An annular, cup-shaped, dust ring 30 of non-magnetic material is secured to the rear surface 32 of the front plate is by any suitable means. The annular dust ring 35 has a central aperture 35 through which the pole piece 25 extends in close fitting relation for the purpose of preventing dust particles from entering.

the outer air gap 29. While the magnet 9, as illustrated in the drawing, is of permanent magnet construction, it could, of course, be of the electro-magnet type.

Attached to the front plate it is a dishpan support 3! for mounting the cone diaphragm i of the loudspeaker. The circumferentially ex-- tending, radially corrugated base portion 33 of the cone l is attached to a flanged portion 35 of the dishpan support 3! by any suitable means. The voice coil form 5, which is attached to the apex end 31 of the cone 5, is supported by a flexible spider or corrugated centering member 39, the purpose of which is to maintain the voice coil centered in the outer air gap 29. The outer periphery 40 of the spider 39 is connected to the dishpan support 3! by any suitable means.

The base portion M of the small cone 3 is circumferentially corrugated similar to the large cone base portion 33 and has its periphery 42; attached by suitable means to the inner surface 45 of the cone 1, thus eliminating the need of a dishpan support for the small cone 3. The apex end 41 of the small cone 3 is attached to a circular disc or center suspension 39 which is securely fastened to a bushing 5i) attached to the end 5| of the center pole piece 21 by means of a screw 52, the disc as maintaining the voice coil 8 centered in the air gap 27!. For conveniently connecting the voice coil 8 of the cone 3 to the output stage of an audio amplifier, there is attached to the cylindrical pole piece 25 a terminal strip 53 which is provided with a pair of terminals 55 to which the small cone voice coil leads 51 are attached. Electrical leads 55 from the audio amplifier output stage extend through apertures 6!, 65 provided in the pole piece 25 and are connected to the terminals 55 on the strip 53.

In order to keep vibrations of the large cone I from being transmitted to the small cone 3, certain structural details must be observed. Reference isv made to the vibrating system mechanical network illustrated in Figure 3, wherein:

f1=the driving force in the low frequency voice coil; I j2=the driving force in the high frequency voice coil; m1=the mass of the low frequency cone and coil; mz=the mass of the high frequency cone and coil; m3.=the mass of air in the vent holes 67! (referred to more particularly hereinafter) in the low frequency cone;

CM1=the compliance of the outer suspension system of the low frequency cone;

CM2=the compliance of the centering suspension of the low frequency cone;

CMs=the compliance of the centering suspension of the high frequency cone;

CM4=the compliance of the outer suspension system of the high frequency cone; and

CM5=the compliance of the cavity 65 (referred to more particularly hereinafter) between the low and high frequency cones.

It Will be observed from this diagram that the above-mentioned objective can be attained by making the compliance Cm of the center suspension 49 of the small cone 3 very small compared to the compliance 0M4 of the small cone outside or base suspension 4!. That is, the center suspension member 49 should be relatively stiff com pared to the base suspension M. In addition thereto, it is also necessary to vent the space 55 between the large cone l and the small cone 3, in order to compensate for the stiffness of the air in the space 65. Thus, by providing a plurality of aperturesor vent holes 67 in the large cone l between its apex end 3! and the point at which the small cone periphery A3 is attached to the large cone inner surface 45, a mass m3 (defined above) is placed in shunt with the compliance CM5 of the cavity 65. The size of the apertures 61 is dependent upon the volume of the cavity or space 65, that is, they should be of a size such that they will provide an impedance which is small compared to the impedance of the cavity or space 65. For the purpose of keeping dust particles from entering the inner air gap 2? through the apertures 6?, suitable screens fill, which may be made of porous, bakelized cloth, are disposed over the apertures till.

From the foregoing description, it Will be apparent to those persons skilled in the art that We have provided an improved loudspeaker which is capable of reproducing signals over an extended. audio frequency range. For example, the response from one loudspeaker, which was constructed in accordance with our present invention'and was provided with a flat baflle having dimensions approximately six feet by six feet, covered a frequency range of approximately 50 to 11,000 cycles per second, as shown by the response frequency characteristic curve ll illustrated in Figure 4 of the drawing. Although we have shown and described but a single modification of our present invention, it will, no doubt, be apparent to those persons skilled in the art that other modifications, as Well as changes in the particular one described above, are possible Within the spirit of our invention. Therefore, we desrie that the particular form of our invention described herein shall be considered as illustrative and not as limiting.

What is claimed is:

1. In a loudspeaker having means for converting electrical oscillations into sound Waves, a plurality of cone shaped diaphragms operatively associated With said means, said diaphragms being mounted for independent vibratory movement and being concentrically arranged, means supporting said diaphragms in spaced apart relation one Within the other, means including said supporting means for enclosin an air space between said diaphragms, and means in one of said diaphragms for venting said air space thereby to prevent vibrations of one of said diaphragms from being transmitted by the air in said space to the other of said diaphragms.

2. The invention set forth in claim 1 wherein said diaphragms are of relatively lesser and greater diameter and particularly effective, respectively, to reproduce higher and lower audio frequencies, and wherein said supporting means includes a compliant coupling attached to the base periphery of said smaller diaphragm, said larger diaphragm and said compliant coupling being attached to the inner surface of said larger diaphragm, and wherein said venting means comprises at least one aperture provided in said larger diaphragm.

3. The invention set forth in claim 2 wherein said aperture is of a size such that it will provide an impedance which is small compared to the impedance of said air space.

6 4. The invention set forth in claim 2 and wherein screen-like material is disposed over said aperture.

HARRY F. OLSON. JOHN PRESTON.

REFERENCES CITED The following references are of record in the file of this patent: smaller diaphragm being disposed within said .0

UNITED STATES PATENTS Number Name Date 1,897,294 Bernard Feb. 14, 1933 2,022,060 Swickard Nov. 26, 1935 2,231,479 Perry Feb. 11, 1941 2,269,284 Olson Jan. 6, 1942 2,426,948 Preston Sept. 2, 1947 

